Field of the Invention
The present invention relates generally to methods of processing a biomass to obtain useful lipid and non-lipid materials, including intermediates for producing biodiesel and ethanol. In particular embodiments, a microorganism is grown on a biodegradable substrate and the resulting biomass is induced to undergo autolysis, thus releasing the lipid and non-lipid materials from the biomass. The present invention is also directed to a processing system useful for practicing the methods of the present invention.
Description of the Related Art
The global energy crisis is continuing to grow as fossil fuels are facing their inevitable depletion. At the same time, the burning of fossil fuels is increasing the production of greenhouse gases. Substituting biofuels for fossil fuels will decrease the greenhouse effect, and with a steady, sustainable, and uninterrupted supply, biofuels will not be a finite fuel source. However, liquid biofuels generated thus far have their own difficulties and concerns that need to be addressed and overcome.
The first generation of liquid biofuels was derived from plants, e.g., starch; cane sugar; and corn, rapeseed, soybean, palm, and other vegetable oils. Therefore, first generation biofuels compete with human and animal nutrition for agricultural crops, because these crops are produced on a limited amount of land. This limited amount of farm land cannot satisfy both global food and fuel needs, thereby generating the food vs. fuel conflict. Analysis has shown that alternative fuel vehicles will not be adopted unless the alternative fuels are made widely available (see, e.g., Struben and Sterman, Environment and Planning B: Planning and Design, 2008, 35(6):1070-1097 and Stauffer, Alternative-Fuel Vehicles: Enabling the Transition, MIT Energy Initiative Spotlight [online], [retrieved on Jul. 11, 2007]. Retrieved from the Internet: <URL: http://web.mitedu/mitei/research/spotlights/alternative-fuel.html>). Accordingly, first generation biofuels are limited by competition with food crops for land and cannot be abundant enough to become widely available, thus preventing the adoption of alternative fuels.
When it became clear that agricultural crops did not represent a steady, sustainable and uninterrupted supply for biofuels, researchers looked for new and abundant materials that contain ample polysaccharides and lipids to produce ethanol and biodiesel, respectively, and that are not in conflict with human and animal nutrition. The materials identified as new fuel sources were cellulose and algae, and the liquid biofuels produced from these materials are regarded as second generation liquid biofuels. Cellulose, or lignocellulose, was selected as the most abundant polysaccharide in nature, with an average polysaccharide content comprising 60% of the dry weight material. Algae were selected to produce lipids for several reasons. In particular, fatty acids constitute up to 40% or more of the overall mass of some types of algae, and algae have a fast rate of growth. In addition, algae consume the greenhouse gas carbon dioxide and produce the globally needed oxygen as a metabolic waste product.
However, the selection of lignocellulose and algae as starting materials introduced a new set of problems. In particular, generating ethanol from lignocellulose has a number of technical problems. Delignification of lignocellulose to liberate cellulose and hemicellulose from their complex with lignin is the rate-limiting and most challenging step in the production of ethanol from lignocellulose (Lin and Tanaka. Appl Microbiol Biotechnol. 2006; 69(6):627-642). In addition, depolymerization of the carbohydrate polymers (e.g., acid hydrolysis of the cellulose from wood) destroys most of the desired materials, simple fermentable sugars, in the process. Also, the fermentation of hexose and pentose sugars together generates a low ethanol yield. Hydrolysis of lignocellulose feedstock via high temperatures, acid treatment, and/or high pressure is also a complex energy consuming process. Enzymatic hydrolysis of pretreated cellulosic biomass with cellulase is a constrained process because the enzyme is inhibited by the hydrolysis products (i.e., glucose and short cellulose chains).
In addition to, and due in part to, the technical problems of generating lignocellulosic ethanol, its production cost is high despite the fact that lignocellulose is an inexpensive agricultural residue when compared to the starch-based agricultural products used in the production of first generation biofuels. Also, the costly initial investment to convert or build a lignocellulose refinery would result in a small number of lignocellulose refineries, high freight costs for delivering bulky cellulose long distances to the limited number of refineries, very large and costly storage areas to contain the biomass, and a large output of residual waste. The cost of lignocellulosic ethanol is not less than the cost of producing the first generation fuel. A study of the European ethanol market determined that the price of lignocellulosic ethanol would break even with that of fossil fuels when the price of oil is €90 ($115) per barrel (Europe's Ethanol Market Potential. Phoenix: Energy Business Reports, 2008). In the United States, a similar study indicated that the price of cellulosic biofuels would break even, without incentives, with that of fossil fuels when the price of oil is $90 per barrel (90-Billion Gallon Biofuel Deployment Study: Executive Summary. HITEC, February 2009 [online], [Retrieved on May 18, 2009]. Retrieved from the Internet: <URL: http://hitectransportation.org/news/2009/Exec_Summary02-2009.pdf>).
The high cost of lignocellulosic ethanol, the drastic drop in oil prices at the end of 2008, and the global economic crisis contributed in putting many lignocellulosic ethanol projects on hold and leading to a reluctance to invest in ethanol projects. Additionally, since the mid-1970s, extensive research has been carried out in the field of lignocellulosic ethanol production; however, the first lignocellulosic ethanol fuel plant was not operational until 2004 in Canada. It is a slowly developing technology, and it is only anticipated to replace one-third of the gasoline used in the United States by the year 2030. The political and environmental needs for alternative fuels are pressing (see, e.g., Milliken. “World has 10-Year Window to Act on Climate Warming—NASA Expert” Reuters, September 2006 [online], [Retrieved on May 18, 2009]. Retrieved from the Internet: <URL: http://www.commondreams.org/headlines06/0914-01.htm>), and the need for alternative fuels will also likely become heightened for economical reasons long before the lignocellulosic ethanol technology is able to meet them (see, e.g., The Rush to Ethanol: Not All Biofuels Are Created Equal [online], [Retrieved on May 18, 2009]. Retrieved from the Internet: <URL: http://www.newenergychoices.org/uploads/RushToEthanol-rep.pdf>).
Although the lipids derived from algae are readily converted to biodiesel by known methods, and lipid extraction via oilseed extraction is available (see, e.g., U.S. Pat. No. 4,456,556), the cultivation of algae is challenging. For example, growing algae in an open-pond system is much less expensive than growing algae in a closed photobioreactor system; however, the open-pond system is open to contaminants, and it is more difficult to provide optimal amounts of carbon dioxide and light while maintaining the temperature and pH to achieve maximum growth of the algae in an open-pond. Thus, stimulating exponential growth of algae in a closed or open system is costly, and cultivating algae on a commercial scale is exceedingly difficult. The industry is still testing a wide variety of methods for growing algae, e.g., open ponds, closed bioreactors, and other processes. Bioreactors have proven to be most effective in producing high quality algae at the greatest rate, but they are expensive, and it has yet to be shown that algae are economically feasible for commercial scale production (see, e.g., Benemann, Opportunities and Challenges in Algae Biofuels Production [online], [Retrieved on Jun. 22, 2009]. Retrieved from the Internet: <URL: http://www.futureenergyevents.com/algae/whitepaper/algae_positionpaper.pdf>).
In view of the limitations associated with the production of first and second generation biofuels, including the food vs. fuel conflict of first generation biofuels, and the high production costs associated with second generation biofuels, there is clearly a need in the art for new methods of efficiently and cost-effectively producing alternative fuels without taxing the environment or competing with food production. The present invention solves this problem by providing an efficient and cost-effective method to produce products useful in biofuels and other products.